The effects of larval instars and ages of Plutellae xylostella at initial parasitization on developmental time, rate of cocoon formation, and parasitic capacity of resultant female wasp of Cotesia plutellae, were observed. The results showed that the development of egg and larva of parasitoid initially parasitizing in the 3rd instar larva was faster than development of those initially parasitizing in the 2 super(nd) and the 4 super(th) instars larvae. The rate of cocoon formation and parasitic capacity of resultant female wasps were also higher in former, indicating that the 3 super(rd) instar larva of P. xylostella was more suitable than the 2 super(nd) and the 4 super(th) instars larvae for C. plutellae, though host age at parasitization had no effect on the emergence rate of parasitoids. In all host instars and ages tested, there were some parasitized individuals that died before the parasitoid larva matured and ate out of the host. For host larvae initially parasitized at the 2 super(nd) and the 3 super(rd) instars, these deaths occurred mainly during larval development; for those larvae initially parasitized at the 4 super(th) instar, many deaths occurred during pupal development. When host larvae were initially parasitized at the 3 super(rd) day of the 4 super(th) instar, 62.5% of parasitized larvae died at the pupal stage. The relationship between suitability of host age and oviposition preference of parasitoid was also discussed. none